The circular dichroism of proteins and nucleic acids responds strongly to changes in secondary and tertiary structure, and allows initial assignment of unknown structures. Circular dichroic spectra may be measured on a microgram of sample, or less, depending on the sensitivity of the spectrometer. As a result, circular dichroic spectra permit the study of ligand binding to proteins and nucleic acids, and of protein/nucleic acid interactions, at physiological concentrations, using microgram quantities of purified samples. Similarly, model structures may be assigned to unknown macromolecules early in their analysis, before sufficient amounts of pure sample have been obtained for NMR spectroscopy or X- ray crystallography. Based on these capabilities, a sensitive, modern, and reliable circular dichroism spectrometer comprises a vital resource for determining the structure and function of proteins and nucleic acids. The core group of users at Jefferson proposes to replace the unreliable 15-year-old Jasco J-500 spectropolarimeter acquired by the PI under RR01554 with a modern, computer-controlled Jasco J-715 spectropolarimeter. The proposed new spectrometer can measure reliably down to 165 nm, rather than approximately 200 nm, with significantly greater signal-to-noise ratio. Peltier temperature control of multiple samples will allow temperature precision of +/-0.2 degrees, compared with circulating water bath control of single samples at +/-2.0 degrees. A modern computer interface will allow multiple scanning with wavelength precision of +/-0.2 nm, rather than +/-2.0 nm. A structural analysis package will permit facile analysis of spectra and deconvolution into structural components. With a modern spectrometer, the core users will be able to elucidate more rapidly the following questions. 1. How does the backbone of chimeric DNA and PNA derivatives control tetraplex formation by dGGGG motifs? 2. How do transposon Tn7 proteins recognize correct DNA sites for transposition? 3. How do unusual nucleic acid modifications alter the structure and properties of novel c-erbB-2 antisense therapeutics? 4. Do inactivating mutations in oncoproteins TCL-1 and MTCP-1 alter their structures? 5. How do proteoglycans reorganize collagen upon binding? 6. How do zinc finger proteins recognize specific RNA targets? 7. Do small peptide antagonists of receptors active in cancer, HIV infection and immunological diseases adopt structures mimicking the structures of the domains from which they were derived?